Photophysical Diversity of Water-Soluble Fluorescent Conjugated

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Photophysical Diversity of Water Soluble Fluorescent Conjugated Polymers Induced by Surfactant Stabilizers for Rapid and Highly Selective Determination of 2,4,6-Trinitrotoluene Traces Naader Alizadeh, Alireza Akbarinejad, and Arash Ghoorchian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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ACS Applied Materials & Interfaces

Photophysical Diversity of Water Soluble Fluorescent Conjugated Polymers Induced by Surfactant Stabilizers for Rapid and Highly Selective Determination of 2,4,6-Trinitrotoluene Traces Naader Alizadeh,* Alireza Akbarinejad and Arash Ghoorchian Department of Chemistry, Faculty of Basic Sciences, Tarbiat Modares University, P.O. Box 14115175, Tehran, Iran KEYWORDS: water soluble conjugated polymers, nanoparticles, surfactant stabilizers, fluorescence quenching, fluorescence sensor, charge transfer. ABSTRACT: The increasing application of fluorescence spectroscopy in development of reliable sensing platforms has triggered a lot of research interest for the synthesis of advanced fluorescent materials. Herein, we report a simple, low-cost strategy for the synthesis of a series of water soluble conjugated polymer nanoparticles with diverse emission range using cationic (hexadecyltrimethylammonium bromide, CTAB), anionic (Sodium dodecylbenzenesulfonate, SDBS) and nonionic (TX114) surfactants as the stabilizing agents. The role of surfactant type on the photophisical and sensing properties of resultant polymers has been investigated using dynamic light scattering (DLS), FT-IR, UV-vis, fluorescence and energy dispersive X-ray (EDS) spectroscopies. The results show that the surface polarity, size, spectroscopic and sensing properties of conjugated polymers could be well controlled by the proper selection of the stabilizer type. The fluorescent conjugated polymers exhibited fluorescence quenching toward nitroaromatic compounds. Further studies on the fluorescence properties of conjugated polymers revealed that the emission of the SDBS stabilized polymer, N-methylpolypyrrole-SDBS (NMPPY-SDBS), is strongly quenched by 2,4,6-trinitrotoluene molecule with a large Stern −Volmer constant of 59526 M-1 and an excellent detection limit of 100 nM. UV-vis and cyclic voltammetry measurements unveiled that fluorescence quenching occurs through a charge transfer mechanism between electron rich NMPPY-SDBS and electron deficient 2,4,6-trinitrotoluene molecules. Finally, the as-prepared conjugated polymer and approach was successfully applied to determination of 2,4,6-trinitrotoluene in real water samples.

1. INTRODUCTION Soluble conjugated polymers present an astonishing class of fluorescent materials which have attracted tremendous popularity, owing to their outstanding physical and spectroscopic properties.1-4 Synthesis of conjugated polymers without any modification usually leads to formation of insoluble products which restricts studying their spectroscopic properties.5-7 A general approach for the preparation of water soluble conjugated polymers is incorporation of hydrophilic moieties to the polymer chains.8 Surfactants provide an efficient class of materials for the synthesis of conjugated polymers with satisfactory water solubility.9, 10 Apart from their hydrophilic nature, surfactants enhance the water solubility of conjugated polymers through electrostatic stabilization concept.11

The outstanding features of water soluble fluorescent conjugated polymers including: non-toxicity12, exceptional photostability13 and excellent brightness14 make them attractive candidates for developing chemosensors,15-17 biosensors18, 19 and even enantioselective sensors.20 Moreover, the increasing interest toward environmentally friendly approaches is another reason for the widespread attention to this nontoxic group of materials21. However, a major drawback of the most existing water soluble fluorescent conjugated polymers is the complicated synthesis procedures which results in their high price and restricts their vast application. Moreover, in the most cases controlling the structural, spectroscopic and sensing properties of resultant conjugated polymers is difficult and even impossible. Therefore, preparation of water soluble fluorescent conjugated polymers using simple,

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Scheme 1 Synthesis of conjugated polymer nanoparticles using cationic, CTAB, anionic, SDBS and nonionic, TX114 surfactants as the stabilizing agents.

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Different types of surfactants were applied for stabilization of polymer nanoparticles during synthesis process (scheme 1). The effect of surfactant type on the photophysical and sensing properties of conjugated polymers was studied. The conjugated polymer stabilized with the anionic surfactant, NMPPY-SDBS, exhibited a considerable fluorescence quenching toward TNT which provided us with a novel, fast, highly selective and sensitive platform for determination of TNT in real water samples. UV-vis and cyclic voltammetry measurements revealed that a charge transfer mechanism between electron rich conjugated polymer and electron deficient TNT molecule enables the sensor platform to selectively respond to TNT in the nM domain. 2. EXPERIMENTAL SECTION

cost-effective and controllable synthesis strategies will open new vistas for the scientists who are working in this field. 2,4,6-trinitrotoluene (TNT) is one of the most widely used nitroaromatics all over the world. TNT residues on the ground can risk the human health and cause serious problems for environment. It has been shown that TNT and its degradation by-products have potential toxicity toward all living organisms including humans, animals and plants.22, 23 Therefore, development of simple, fast and cost- effective methods for determination of trace contents of TNT has long been a subject of interest in different fields of applied science. Several methods have been developed for determination of TNT, including electrochemistry,24, 25 chromatography,26 quartz crystal microbalance,27 colorimetry,28 surface enhanced Raman scattering,29 chemiluminescence,30 photoluminescence31 as well as hybrid methods.32 Among the aforementioned techniques fluorescence based methods provide very sensitive and fast responses as well as simple strategies for determination of TNT.33, 34 In the most fluorescence probes developed for determination of TNT, the response results from the direct interaction of fluorescent material and TNT molecules. Therefore, a number of luminescent materials capable of forming strong interactions with TNT molecules have been developed to be used as the sensing element, including, conjugated polyelectrolytes,35 quantum dots,36-38 metal organic frameworks,39, 40 gold clusters41 and nanoclusters,42 carbon materials,43, 44 and functionalized polymer nanofibers45 and nanonets.46 Herein, we describe simple and cost-effective synthesis of a series of water soluble conjugated polymer nanoparticles with different fluorescence emission and excitation range based on N-methylpolypyrrole (NMPPY).

2.1. Synthesis of conjugated polymers. Conjugated polymers were synthesized using a general procedure reported in our previous work.47 For the synthesis of CTAB stabilized polymer, NMPPY-CTAB, N-methylpyrrole (0.72 mmol) was added to 8.75 mL of an acetonitrile–water (80-20) solvent containing CTAB (0.15 mmol) as the stabilizing agent. Then, 1.25 mL of acetonitrile–water solvent containing ammonium persulfate (0.72 mmol) was added to the above solution and stirred for 7 h at 60 ºC to complete polymerization reaction. The undesired inorganic components were separated from polymer nanoparticles by addition of 5 mL chloroform which resulted in transfer of polymer nanoparticles to chloroform phase. After removing the polymer rich phase, chloroform was evaporated to obtain a solid product. For the preparation of TX114 stabilized polymer, NMPPY-TX114, TX114 (0.15 mmol) was used as the stabilizing agent. The same synthesis procedure was used for the preparation of SDBS stabilized polymer, NMPPY-SDBS, except SDBS (0.15 mmol) was used as the stabilizer and separation of the polymer from undesired components was performed by saturating the aqueous-chloroform mixture with sodium chloride salt. 2.2. Materials Characterization. Fluorescence spectra of conjugated polymers were recorded on a Perkin-Elmer model LS 50B spectrofluorimeter (PerkinElmer, U.K.), using emission and excitation band passes of 10 and 10 nm and a scan rate of 100 nm min-1. UV-vis absorption spectra were recorded using a Model Scinco UV S-2100. Energy dispersive X-ray spectroscopy (EDS) measurements were carried out on a Phenom ProX scanning electron microscope (Phenom-World, Netherlands). Particle size and zeta potential measurements were performed on a Zetasizer Nano ZS and submicrometer particle size analyzer (Malvern Instruments Ltd, UK). An ESCA Lab220I-XL spectrometer equipped with an Al Kα X-ray radiation source (hν = 1486.6 eV) which operates at a vacuum of ˂ 10-7 Pa was used to measure the XPS spectra of the conjugated polymers. Gel permeation chromatography (GPC) measurements were performed using a Knauer instrument. All cyclic voltammetry measurements were

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Fig. 1 (a) Size distribution and zeta potential profile (the inset spectra) obtained by DLS in water, (b) Fluorescence emission and excitation spectra (the inset spectra) in water, (c) Relative fluorescence response of polymer nanoparticles to 50 µM of different chemical groups in water, the concentration of TNT for NMPPY-SDBS nanoparticles is 10 µM. NMPPY-SDBS (green), NMPPY-TX114 (purple) and NMPPY-CTAB (blue).

performed on an AUTOLAB model PGSTAT30 (Metrohm, Netherlands) and glassy carbon, Pt and Ag/AgCl as working, counter and reference electrodes, respectively. 2.3. Assay of TNT. 2 mL of 2 mg L-1 aqueous solution of NMPPY-SDBS was titrated by adding a given concentration of TNT standard solution in methanol and then the fluorescence spectra were recorded over the wavelength range of 300 −600 nm at an excitation wavelength of 305 nm at room temperature. The plot of fluorescence intensity changes vs. TNT concentration showed a good linearity at the maximum emission wavelength of 390 nm. Well water collected from our university in Tehran, Iran and Persian Gulf water samples were used to evaluate the applicability of NMPPY-SDBS for the real sample analysis. Before analysis the water samples were filtered through a 0.45 μm pore size filter to remove any sediment or suspended particles. Quantitative analysis of the water samples was performed by standard addition method to reduce the matrix effect. NMPPY-SDBS was added to 2 mL of each water samples to obtain a concentration of 2 mg L-1 and then the samples were spiked with standard TNT solution and the fluorescence spectra were recorded at an excitation wavelength of 305 nm. 3. RESULTS AND DISCUSSION 3.1. Photophysical and sensing properties of conjugated polymers. Polymerization of N-methylpyrrole in the presence of surfactants could be lead to the attachment of surfactants to the conjugated polymers. EDS technique was applied to investigate the attachment of surfactants to the polymers. Fig. S1 shows the weight percentage of different elements for all synthesized polymers. The attachment of SDBS, TX114 and CTAB surfactants could be well explored by measuring the amount of Na, O and Br elements in the polymer structures, respectively. As seen in Table 1 and Fig S1, NMPPY-SDBS has a high content of Na, while NMPPY-TX114 and NMPPY-CTAB are rich in O and Br, respectively which shows that all anionic, nonionic and cationic surfactants have been successfully attached to the polymer structures. The size and surface charge (zeta

potential) of the conjugated polymers could be affected by the attachment of surfactants. The synthesized conjugated polymers were subjected to dynamic light scattering (DLS) analysis to measure the particle sizes and zeta potentials. The Size distribution and zeta potential profile of NMPPY-SDBS, NMPPY-TX114 and NMPPY-CTAB are shown in Fig 1a. The results of size and zeta potential measurements are also summarized in Table 1. As seen, the particle size of the polymer nanoparticles is sensitive to the type of surfactant used. Using the nonionic surfactant, TX114, triggers the formation of the largest polymer nanoparticles, 55 nm, while anionic and cationic surfactants produce polymer nanoparticles of 28 and 32 nm, respectively. Interestingly, we found a relation between the size of polymer nanoparticles and their zeta potential values. NMPPY-SDBS shows a large negative zeta potential of -44.1 which causes a considerable electrostatic repulsion between polymer nanoparticles and promotes the formation of small nanoparticles. On the other hand, the zeta potential of the polymer stabilized with the nonionic surfactant, NMPPY-TX114, is too low to cause enough electrostatic repulsion between polymer nanoparticles, resulting in the formation of larger nanoparticle. Moreover, NMPPY-CTAB nanoparticles possess a large positive zeta potential of 33.3, which triggers the formation of polymer nanoparticles as small as 32 nm. The results obtained by DLS measurements suggest that dissociation of ionic surfactants attached to the surface of polymer nanoparticles leads to the formation of charged polymer nanoparticles which subsequently stabilizes the nanoparticles and protects them from aggregation through electrostatic repulsion concept. The surface polarity of the polymer nanoparticles is also dependent on the surfactant type. In the synthesis process when chloroform was added to separate polymers from undesired inorganic components, NMPPY-TX114 immediately transferred into chloroform, NMPPY-CTAB transferred after shaking the mixture and letting to stand for 10 min and NMPPY-SDBS only transferred after saturating the mixture with sodium chloride. TX114 is a nonionic surfactant, therefore it is expected that

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Table 1 Experimental results of the characterization of different conjugated polymers. Polymer

(Weight %)

Size/ nm

Zeta Pot/ mV

λex

λem

Na

O

Br

NMPPYSDBS

49.6

20.2

0.0

28

-44.1

305

390

NMPPYTX114

0.0

47.7

1.3

59

1.7

309

397

NMPPYCTAB

0.0

32.1

20.5

33

33.3

348

431

NMPPY-TX114 would have a less polar character compared to NMPPY-CTAB and NMPPY- SDBS, resulting in high affinity of this polymer for nonpolar chloroform solvent. It is worth pointing out that NMPPY-TX114 has also good solubility in water which is due to hydrophilic polyethylene oxide chain of TX114 which forms hydrogen bonds with H2O molecules. On the other hand, SDBS is an anionic surfactant which dissociates to Na+ and DBS- ions in water. The strong solvation of Na+ ions by water molecules (ΔH°hyd = -409 kJ mol-1)48 make the NMPPY-SDBS to have very good solubility in water. However, after saturation with sodium chloride salt the dissociation degree reduces, giving rise to transfer of NMPPY-SDBS to chloroform. The more facile transfer of NMPPY-CTAB into chloroform compared to NMPPY-SDBS is due to the less hydration of Br(ΔH°hyd = -347 kJ mol-1)48 than Na+ ions which gives NMPPY-CTAB a less polar character. As a result the following order could be proposed for the polarity of the conjugated polymers: NMPPY-SDBS˃ NMPPY-CTAB˃ NMPPY-TX114. In the next step, we investigated the effect of surfactant type on the emission properties of the conjugated polymers. Fig. 1b shows the fluorescence emission and excitation spectra of the conjugated polymers. The values of the maximum excitation and emission wavelengths of the conjugated polymers are summarized in Table 1. We found that the position of excitation and emission peaks of the polymers is consistent with the charge of surfactants used for stabilization, so that the polymer stabilized with the anionic surfactant, NMPPY-SDBS, gives the peaks in the lowest wavelength region while the peaks of the polymer stabilized with the cationic surfactant, NMPPY-CTAB, are in the highest wavelength region. The excited state of the conjugated polymers has a larger dipole moment than their ground state. Water is a polar solvent, therefore, if interactions of surfactants with the polymers in water lower the dipole moment of the excited state, then the excited state will be raised to a higher energy level and the energy band gap of the polymer will increase, which in turn results in a blue shift in the emission and excitation spectra of the conjugated polymer. Alternatively, those interactions that increase the dipole moment of the excited

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state will result in a red shift in the excitation and emission spectra of the conjugated polymer. Hence, the observed red shift of NMPPY-CTAB and blue shift of NMPPY-SDBS could be attributed to an increase and decrease in dipole moment of excited state of the conjugated polymers by CTAB and SDBS surfactants, respectively. The N-methyl unit of the conjugated polymers is an electron donating group, giving the polymers a high electron density. Therefore we expected a good interaction between the electron rich conjugated polymers and compounds with high electron affinity. The potential interaction of the synthesized conjugated polymers with different chemicals was examined by studding the effect of compounds with different chemical groups on the fluorescence of the conjugated polymers at their maximum emission wavelength. Fig. 1c illustrates the fluorescence response of the conjugated polymers to different chemical groups including: amine (n-butylamine), alcohol (methanol), ketone (acetone), thiol (propanethiol), nitrile (acetonitrile), phosphonate (dimethylmethylphosphonate, DMMP), nitro (nitromethane, NM), nitroaromatic (TNT, nitrobenzene, NB and 1,3-dinitrobenzene, DNB) and nitroamine (RDX). The results show that the type of surfactant used for stabilization has a considerable impact on the sensing properties of the resultant conjugated polymers. As expected, the fluorescence of all conjugate polymers was quenched by electron deficient nitroaromatic compounds but with different intensities. The polymer stabilized with anionic surfactant, NMPPY-SDBS, gave the highest response while the minimum response was obtained from the polymer synthesized in the presence of cationic surfactant, NMPPY-CTAB. The negative charge of the anionic surfactant elevates the electron density of the polymer, resulting in enhanced interaction of electron rich NMPPY-SDBS and electron deficient nitroaromatic compounds. On the other hand, the positive charge of the cationic surfactant lowers the electron density of the polymer which leads to the weak interaction of the NMPPY-CTAB with nitroaromatics. Due to the highly sensitive response of NMPPY-SDBS to nitroaromatics, especially TNT, we focused our studies on further characterization of this polymer and investigating its fluorescence sensing properties. 3.2. Further characterization of NMPPY-SDBS. The chemical composition of the NMPPY-SDBS was investigated by X-ray photoelectron spectroscopy (XPS). Fig. 2a depicts the survey spectrum of NMPPY-SDBS. The observation of S2p peak at 169.0 eV shows the attachment of the anionic surfactant to the polymer structure. The S2p peak could be decomposed into two components at 169.0 eV and 170.1 eV which are due to the SO3- group of anionic surfactant and an oxidized form of SO3- (Fig. 2b).49, 50 The N1s peak centered at the binding energy of 403.1 eV is ascribed to the N-methyl group of the polymer. It has three components centered at binding

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Fig. 2. XPS spectra of NMPPY-SDBS (a) Survey spectrum, (b) S2p, (c) N1s, (d) O1s, (e) C1s. (f) UV-vis spectrum of NMPPY-SDBS; the inset photographs are NMPPY-SDBS under visible (left) and UV light (right) in water.

energies of 401.8, 402.5 and 408.8 eV which are attributed to neutral amine and positively charged amine groups, polarons and bipolarons, respectively (Fig. 2c).51 The O1s peak at 535.4 eV could be deconvoluted into two components occurring at 534.7 and 535.4 eV which originate from sulfate group of the anionic surfactant and C–O interactions in the polymer backbone (Fig. 2d).47 The C1s peak occurring at 285.5 eV is decomposed into four components corresponding to aromatic carbons of anionic surfactant (284.9 eV), aliphatic carbons of anionic surfactant and polymer (285.5 eV), α-carbons of the pyrrole cycle (286.2 eV) and the C-O interactions in the polymer backbone (287.1 eV) (Fig. 2e).49 The elemental composition of the NMPPY-SDBS polymer was determined by XPS results. Also, the molecular weight of NMPPY-SDBS, determined by GPC, was found to be 17400 g mol-1. Finally, the characterization results were used to establish the chemical structure of NMPPY-SDBS (Fig. S2). All conjugated polymers synthesized in this work are completely soluble in water, so that no precipitation was observed after storing for more than 6 months at room temperature. The water solubility of NMPPY-SDBS was confirmed by UV-vis spectroscopy, as depicted in Fig. 2f. The strong absorption band which spans the wavelength range from 190 nm to 400 nm corresponds to the π-π* transitions of pyrrole ring. When NMPPY-SDBS was exposed to a UV lamp, it displayed a bright and efficient blue fluorescence, as showed in the inset of Fig. 2f. The strong quenching of the blue fluorescence of NMPPY-SDBS by TNT is the subject of our further studies.

3.3 Determination of TNT. Before studding the sensing properties of NMPPY-SDBS toward TNT, the stability and pH dependency of NMPPY-SDBS emission was investigated. A fluorescent material should be stable enough to be used as a fluorescence probe. Therefore, the photostability of NMPPY-SDBS in water was studied for more than 150 min. The results of fluorescence stability measurements have been presented in Fig. S3. As seen, no decrease in fluorescence intensity was observed, suggesting the excellent photostability of NMPPY-SDBS which is crucial for fluorescence sensing applications. The influence of solution pH affecting the fluorescence properties of NMPPY-SDBS was studied over the pH range between 3-11. According to Fig. S4, the fluorescence intensity remained constant within the 5—11 pH range, while a slight decrease in fluorescence intensity was observed when pH was reduced to 3, which could be ascribed to protonation of nitrogen atoms of pyrrole rings in strong acidic solution. Therefore, pH 7.0 was chosen for the further studies. The fluorescence quenching of luminescent materials by means of nitroaromatic compounds has been the subject of many studies.52 In most cases the electron rich fluorophore interacts with electron withdrawing nitroaromatic compounds through a charge transfer process, which finally leads to fluorescence quenching of fluorophore. Two types of quenching mechanisms have been recognized, static and dynamic. Static quenching refers to a process in which fluorescence intensity decreases as a result of the formation of a reversible complex between the fluorophore and quencher molecules in the ground state. Dynamic quenching on the other hand

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Fig.3 (a) Fluorescence quenching of NMPPY-SDBS upon addition of different concentrations of TNT (0.25, 0.50, 0.99, 1.97, 3.90, 6.73, 9.48, 13.02, 17.28, 22.30, 28.60 and 34.86 µM); the inset photographs illustrate the fluorescence quenching of NMPPY-SDBS with addition of TNT. (b) Stern–Volmer plots for TNT, DNB and NB. (c) Cyclic voltammograms of 100 mg L-1 NMPPY-SDBS, 100 µM TNT and mixture of 100 mg L-1 NMPPY-SDBS and 100 µM TNT in water containing 0.1 M LiClO4 at a scan rate of 100 mV s-1 versus Ag/AgCl.

is the process of complex formation between a fluorophore and a quencher in the excited state. The systems which undergo a static or dynamic quenching could be described by the well-known Stern-Volmer equation:53 𝐹𝐹0 𝐹𝐹

= 1 + 𝐾𝐾𝑆𝑆𝑆𝑆 [A]

Where F and F0 are the fluorescence intensities in the presence and absence of the quencher, [A] is the molar concentration of the quencher and KSV is the quenching constant (M-1). The fluorescence quenching of NMPPY-SDBS in the presence of TNT is illustrated in Fig. 3a. Upon addition of TNT the fluorescence intensity of NMPPY-SDBS was instantly quenched, so that a quenching of 71% was achieved at the concentration level of 35 µM. The fluorescence quenching is also evident from the photographs of NMPPY-SDBS probe illuminated with a UV lamp, in the absence and presence of different concentrations of TNT (The inset photographs of Fig. 3a). Fig. 3b shows the Stern-Volmer plots for quenching of the fluorescence of NMPPY-SDBS by NB, DNB and TNT. The fitting of the Stern-Volmer plots resulted in quenching constants of 5303, 9574 and 59526 M-1 for NB, DNB and TNT, respectively. As the results show, there is a direct relationship between the quenching constant and the number of nitro groups linked to aromatic ring which results from the strong electron withdrawing effect of nitro group. Moreover, the quenching constant of TNT obtained in this work was found to be among the highest values known for TNT fluorescence sensors, indicating the substantial potential of NMPPY-SDBS to form a super-quenching system toward TNT. The fluorescence quenching of NMPPY-SDBS by TNT shows linearity in the concentration range of 0.25- 34.86 µM with an excellent detection limit of 100 nM. As seen in Table S1, the detection limit of NMPPY-SDBS is better than most of the previously reported TNT fluorescence sensors in aqueous phase. The quenching constants for NB, DNB and TNT obtained from Stern-Volmer plots suggest that electron

deficient nitroaromatic compounds quench the fluorescence of NMPPY-SDBS through a charge transfer process which leads to the formation of a charge transfer complex between the nitroaromatic compounds and NMPPY-SDBS. The existence of methyl group on the pyrrole ring and also anionic surfactants on the nanoparticles surface increase the electron density of NMPPY-SDBS and facilitate the charge transfer process between the conjugated polymer and TNT. UV-vis spectroscopy is a common technique to study the charge transfer interactions. Fig. S5 shows the UV-vis spectra of different concentrations of TNT in the presence and absence of NMPPY-SDBS. As seen, no new absorption peaks were appeared upon addition of TNT to NMPPY-SDBS solution. Nevertheless, the plot of absorbance changes at 335 nm vs. TNT concentration indicated different slopes in the presence and absence of NMPPY-SDBS which confirms the formation of a charge transfer complex between NMPPY-SDBS and TNT (Fig. S6). The results obtained by UV-vis measurements suggest that fluorescence quenching accrues through a static quenching mechanism. TNT is an electrochemically active compound, and therefore comparing the electrochemical behavior of it in the presence and absence of NMPPY-SDBS will provide us with further information about the mechanism of fluorescence quenching. Therefore, cyclic voltammetry technique was used to investigate the mechanism of fluorescence quenching. Fig. 3c shows the cyclic voltammograms of 100 mg L-1 NMPPY-SDBS, 100 µM TNT and 100 mg L-1 NMPPY-SDBS in the presence of 100 µM TNT in water. As expected, the voltammogram of NMPPY-SDBS showed no oxidation or reduction peaks, indicating very good electrochemical stability of NMPPY-SDBS polymer. When TNT was added to the electrochemical cell three reduction peaks were observed for TNT at potentials of -1.036, -0.912 and -0.710 V. In the next step, the voltammogram of TNT was recorded in the presence of NMPPY-SDBS to gain more information on the type of interaction between TNT and NMPPY-SDBS. The reduction process of TNT became

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Table 2 Recovery of TNT in real water samples using NMPPY-SDBS fluorescence sensor. (n=3) Sample Persian Gulf water

Well water

Spiked amount (µM) 0.00 1.00 10.00 30.00 0.00 1.00 10.00 30.00

Found amount (µM) Not detected 1.01 (±0.08) 9.07 (±0.53) 26.09 (±1.74) Not detected 1.05 (±0.05) 9.78 (±0.84) 27.27 (±2.26)

Recovery (%) 101.3 90.7 87.0 105.4 97.8 90.9

more facile in the presence of NMPPY-SDBS and the peaks were shifted to -0.794, -0.663 and -0.470 V. This observations confirm the UV-vis results and show that a charge transfer process takes place between TNT and NMPPY-SDBS in which electron deficient TNT molecule serves as electron acceptor and electron rich NMPPY-SDBS serves as electron donor. To further demonstrate the practical application of the NMPPY-SDBS sensor, we considered applying the proposed method to the detection of TNT in well and Persian Gulf water samples. The standard addition method was used for the analysis of real water samples and the results are summarized in Table 2. We didn’t found TNT in either samples. As could be seen in Table 2, very good recovery values were obtained, suggesting the applicability of the proposed method in determination of TNT in real aqueous media. 4. CONCLUSIONS In summary, a series of water soluble fluorescent conjugated polymers were synthesized using cationic, anionic and nonionic surfactants as the stabilizing agents. The results show that the photophysical and sensing properties of the conjugated polymers could well be controlled by the proper choose of stabilizer type. Due to its high electron density, the fluorescence intensity of NMPPY-SDBS is quenched significantly by electron deficient TNT molecules. The present sensor platform enables determination of TNT in the nM domain. UV-vis and cyclic voltammetry studies revealed that a charge transfer process between NMPPY-SDBS and TNT triggers the static fluorescence quenching of NMPPY-SDBS. The developed NMPPY-SDBS fluorescence sensor offers excellent features including: low-cost, easy synthesis, complete water solubility, good photostability and a fast, selective and sensitive response to TNT with applicability to real water sample analysis. The results of this work will open up opportunities for design of new photoluminescence-based chemo- and biosensors with controlled physical and spectroscopic properties.

ASSOCIATED CONTENT Supporting Information

Weight percentage of different elements in conjugated polymers measured by EDS analysis, Chemical structure of NMPPY-SDBS polymer, Photostability measurements for NMPPY-SDBS polymer, Fluorescence emission spectra of NMPPY-SDBS at various pH values, UV-vis spectra of different concentrations of TNT in the absence and presence of NMPPY-SDBS in chloroform, The changes in the absorbance of a chloroform solution at 335 nm upon addition of different concentrations of TNT in the presence and absence of NMPPY-SDBS and a table comparing the LOD of NMPPY-SDBS sensor and other TNT fluorescence sensors. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for the financial support from the Tarbiat Modares University Research Council and the Iran National Science Foundation (INSF).

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Photophysical Diversity of Water-Soluble Fluorescent Conjugated

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